THE INVENTION
Briefly, the sensor is constructed of a frame of monocrystalline silicon and a monocrystalline doped structure is formed on the dielectric diaphragm. The structure is supplied with electrical terminals or terminal connections. This structure can be used in two modes. When used as a temperature-dependent resistor, the silicon structure supported on the dielectric diaphragm is doped with deep doping materials having an energy level which is at least approximately at the center of the energy gap of silicon. When used as a voltage source, that is, employing the Seebeck effect, the doping materials can be as customarily used in semiconductor technology. In accordance with a feature of the invention, the sensor is made by inserting a silicon structure of the opposite conductivity type to the basic wafer which also forms the frame, and a thin dielectric layer is formed on the surface of the wafer. Openings are cut into the dielectric layer to provide connection terminals and a cut-out is etched into the silicon wafer which extends as far as the dielectric layer, or as far as the silicon structure, to form the sensor.
In contrast to the above, the arrangement in accordance with the invention has the advantage that a structure of monocrystalline silicon is used for temperature measurement. Because monocrystalline silicon potentially has a very large Seebeck effect or high sensitivity of conductivity with respect to temperature, very sensitive temperature sensors can be produced by means of monocrystalline silicon.
Good thermal separation of the monocrystalline silicon structure from the frame is achieved by disposing silicon nitride, silicon oxide or silicon oxinitride on a dielectric diaphragm. The top surface is covered with a protective dielectric layer as a protection against dirt and damage.
A particularly smooth surface of the diaphragm is achieved by disposing the monocrystalline silicon structure on the underside of the frame. In this case, the monocrystalline silicon structure is particularly simply connected through openings in the diaphragm. Different manufacturing processes must be used when the monocrystalline silicon structure is to be located on top of the diaphragm. Because of this it is not necessary to cut openings for connecting the silicon structure. The temperature sensitivity of the electrical resistance of monocrystalline silicon can be greatly increased by the use of deep doping materials. It is possible, in comparison with flat doping materials, to attain a sensitivity increase by a factor of 1000. Gold, for example, is suitable for deep doping.
The temperature sensor is designed so that it makes use of the Seebeck effect by the partial disposition of the mono-crystalline silicon structure in the area of the frame. appropriate feed lines have contact points located partially in the area of the frame and partially in the area of the diaphragm. The Seebeck effect has the advantage that no external voltage needs to be provided for measuring the temperature. The Seebeck effect is the basis of all thermocouples. The temperature sensor can be used as a mass flow sensor by providing a heater which maintains the diaphragm at a higher temperature than the frame. Through measurement of the temperature of the diaphragm it is possible to measure the heat loss of a medium flowing along the top of the diaphragm and thus the amount of the medium flowing by.
A particularly simple manufacture of the temperature sensors is possible by means of the process of the invention. This process can be used in particular for parallel mass production of the temperature sensors of the invention. A particularly exact and reproducible etch stop is achieved by the application of a voltage to the silicon structure. Production of dielectrical layers by chemical reaction of the wafer surface with a gas is particularly simple, separation of the dielectric layers by sputtering or separation from the gas phase makes possible increased freedom in the choice of the materials or decreased temperature stresses of the wafer. Another production method for the temperature sensors of the invention makes use of implantation of oxygen in a silicon wafer. The possibilities for the manufacture of temperature sensors are increased by this process.